I also want Tifa to be a guest. She's a DOA-type of female character, and she being a melee fighter.
Some ideas:
Chun Li or Laura. Laura is more curved or at least shows more skin, but Chun Li is a lot more well-known. I hope her thighs won't be too big though. And no need for her Kikkoken.
Bayonetta. She can punch (have the guns removed) and kick. She has some nice kick combos. But Nintendo currently owns her, and since there's no DOA6 on Switch, never gonna happen.
Kassandra from Assassin's Creed Odyssey. She's a Spartan so can fight for sure. She has her Spartan kick. She also looks good. Could be up to DOA standards of beauty.
Nina. Or Anna. DOA6 Nina could like Sarah for DOA5. Strong blonde (Nina). Palm-kick combos plus grab. Could do combos like Christie. Both characters are also known to be strong fighters.
Hibana from Sega's Kunoichi/Nightshade. She's a ninja. That's all I know. Or perhaps Hotsuma from Shinobi.
Some wild ideas:
Jackie Chan. He's a well-known martials arts actor who uses the environment to fight groups of enemies. Not sure how it would work in DOA.
The female Shepard from Mass Effect 3. I don't know anything about her and the series. If DOA4 has Nicole/Spartan-458, a shooter without a gun, maybe DOA6 can have the same.
Sanji from One Piece. He can only kick. His punch buttons will only do very small damage, like chip damage. And he won't kick the DOA ladies. Very useless fighter.